Time-reversal symmetry breaking in the superconducting state of ScS

We have studied the electronic properties of ScS, a transition-metal monochalcogenide with a rocksalt crystal structure, using magnetization, specific heat, transport, and muon spin rotation/relaxation ($\mu \mathrm{SR}$) measurements. All measurements confirm the bulk superconductivity in ScS with a transition temperature of $T_C=5.1\left(5\right)$ K. Specific heat together with transverse-field $\mu \mathrm{SR}$ measurements indicate a full gap, while our zero-field $\mu \mathrm{SR}$ study reveals the presence of spontaneous static or quasistatic magnetic fields emerging when entering the superconducting state. We discuss various possible microscopic origins of the observed time-reversal symmetry breaking. As none of them can be readily reconciled with a conventional pairing mechanism, this introduces ScS as a candidate material for unconventional superconductivity.

Figure 1

(a) Powder XRD of ScS obtained at room temperature, shown by open red circles. The solid blue line is the Rietveld refinement whereas the black bars show Bragg reflection peaks. Inset: Crystal structure of ScS. (b) Temperature dependence of the magnetic moment shows $T_C$ at 5.1 K for ScS. The data were collected in an applied field of 1 mT via ZFCW and FCC protocols. The inset shows the enlarged view of the $\rho \left(T\right)$ data, with superconducting transition at $T_{C,\text{onset}}=5.1\left(2\right)$ K. (c) Normalized specific heat $C_{\text{el}}/{\gamma }_{n}T$ is fitted using a single-gap $s$-wave model represented by the solid blue line. Inset: Temperature dependence of the total specific heat in zero field exhibiting $T_C$ at 4.8(1) K.

Figure 2

(a) Transverse-field time domain spectra collected at 6.5 and 0.3 K in an applied magnetic field of 40 mT. The solid lines are fits using Eq. (1). (b) Probability distribution of magnetic field for $T<T_C$ and $T>T_C$. (c) Temperature dependence of ${\sigma }_{\text{sc}}$ (squares), where solid lines represent fitting using Eq. (2) which employs $s$-, $p$-, and $d$-wave models.

Figure 3

(a) Asymmetry spectra recorded at two different temperatures, 0.3 and 8.0 K, in zero magnetic field (small open circles); data at a small longitudinal field (30 mT) are shown by large solid circles. The solid lines are the corresponding fits to the data using Eq. (3). (b) shows the temperature dependence of the relaxation rate $\mathrm{\Lambda }$ with a guide to the eye line, featuring an increase below the superconducting transition temperature $T_C$.